Soil water repellency

This photograph was taken a few days after a forest fire in a pine-forested area very close to Montellano (Sevilla, SW Spain), while planning some field experiments. In the image, several water drops are resting on a water-repellent soil layer without infiltrating.
The fire affected between 70 and 80 ha of a hardly accessible area, characterized by a pine dense forest, shallow soils and steep slopes. Originally an arson fire due to local struggles, fire quickly climbed the northern slope of the mountain, reached the top and moved down the southern slope in hours. The area is used by residents for recreative activities.
Water repellency is a property of soils which, until relatively recent times, has not received much attention. However, its geomorphological consequences are extremely important, since it limits water infiltration in soils, increasing runoff rates and enhancing soil erosion risk. Vegetation (tree species such as pines or eucalyptus, or shrubs such as heather), fungi and other soil midroorganisms are the main sources of hydrophobic organic substances in soils. Climate, soil texture and structure may also modulate the degree of hydrophobicity. Forest fires are a major cause of soil water repellency. During the combustion of litter and soil organic matter, organic substances from soil volatilize and escape as part of the smoke. However, another part can be displaced in depth and condense following a hot-to-cold gradient. These organic substances form hydrophobic coatings on the surface of soil particles and aggregates. So, when the soil reaches temperatures between 200 and 250 °C, water repellency can be increased or induced. If temperature peaks are higher, combustion and volatilization are more intense, and hydrophobic substances may be completely lost or destroyed.
Therefore, soil scientists use soil water repellency as an index of fire severity. A simple way to determine the degree of water repellency is to measure the time required for infiltration of water drops. As in this case, the development of a water-repellent soil layer together with the loss of vegetation cover that protects soil may trigger soil erosion processes in sensitive areas just to get the first storms of the wet season.
I shot the picture with a Canon EOS 350D, 0.6x/58 mm lens, f/10, exposure 1/400 s, ISO-400, and 55 mm focal length, equipped with UV filter.